Laser Imaging System Screens Nanotubes

Photonics.comNov 2010
WEST LAFAYETTE, Ind., Nov. 22, 2010 — Researchers have demonstrated a new imaging tool for rapidly screening structures called single-wall carbon nanotubes, possibly hastening their use in creating a faster and more energy-efficient class of computers and electronics.

The semiconducting nanostructures might be used to revolutionize electronics by replacing conventional silicon components and circuits. However, one obstacle in their application is that, unavoidably, metallic versions form during the manufacturing process, contaminating the semiconducting nanotubes.

Now researchers have discovered that an advanced imaging technology could solve this problem, said Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University.

Metallic and semiconducting single-wall carbon nanotubes are distinguished using a new imaging tool for rapidly screening the structures. The technology may hasten the use of nanotubes in creating a faster and more energy-efficient class of computers and electronics than those in use today. (Image: Weldon School of Biomedical Engineering, Purdue University)
“The imaging system uses a pulsing laser to deposit energy into the nanotubes, pumping the nanotubes from a ground state to an excited state,” he said. “Then, another laser called a probe senses the excited nanotubes and reveals the contrast between metallic and semiconductor tubes.”

The technique, called transient absorption, measures the “metallicity” of the tubes. The detection method might be combined with another laser to zap the unwanted metallic nanotubes as they roll off of the manufacturing line, leaving only the semiconducting tubes.

Findings are detailed in a research paper appearing online this week in the journal Physical Review Letters.

Single-wall nanotubes are formed by rolling up a one-atom-thick layer of graphite called graphene, which could eventually rival silicon as a basis for computer chips. Researchers in Cheng’s group, working with nanomaterials for biomedical studies, were puzzled when they noticed that the metallic nanoparticles and semiconducting nanowires transmitted and absorbed light differently after being exposed to the pulsing laser.

Then researcher Chen Yang, a Purdue assistant professor of physical chemistry, suggested the method might be used to screen the nanotubes for nanoelectronics.

“When you make nanocircuits, you only want the semiconducting ones, so it’s very important to have a method to identify the metallic nanotubes,” Yang said.

The paper was written by Purdue physics doctoral student Yookyung Jung; biomedical engineering research scientist Mikhail N. Slipchenko; Chang-Hua Liu, an electrical engineering graduate student at the University of Michigan; Alexander E. Ribbe, manager of the Nanotechnology Group in Purdue’s department of chemistry; Zhaohui Zhong, an assistant professor of electrical engineering and computer science at Michigan; and Yang and Cheng. The Michigan researchers produced the nanotubes.

Semiconductors such as silicon conduct electricity under some conditions but not others, making them ideal for controlling electrical current in devices such as transistors and diodes.

The nanotubes have a diameter of about 1 nm, or roughly the length of 10 hydrogen atoms strung together, making them far too small to be seen with a conventional light microscope.

“They can be seen with an atomic force microscope, but this only tells you the morphology and surface features, not the metallic state of the nanotube,” Cheng said.

The transient absorption imaging technique represents the only rapid method for telling the difference between the two types of nanotubes. The technique is “label free,” meaning it does not require that the nanotubes be marked with dyes, making it potentially practical for manufacturing, he said.

The researchers performed the technique with nanotubes placed on a glass surface. Future work will focus on performing the imaging when nanotubes are on a silicon surface to determine how well it would work in industrial applications.

“We have begun this work on a silicon substrate, and preliminary results are very good,” Cheng said.

Future research also may study how electrons travel inside individual nanotubes.

1. A constituent part. It may consist of two or more parts cemented together, or with near and approximately matching surfaces. 2. The projection of a vector on a certain coordinate axis or along a particular direction. 3. In a lens system, one or more elements treated as a unit. 4. An optical element within a system.

A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.

That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.

Also known as ground level. The lowest energy level of an atom or atomic system. A material in the ground state is not capable of emitting optical radiation. All other states are called excited states.

Acronym for profile resolution obtained by excitation. In its simplest form, probe involves the overlap of two counter-propagating laser pulses of appropriate wavelength, such that one pulse selectively populates a given excited state of the species of interest while the other measures the increase in absorption due to the increase in the degree of excitation.

An electronic device consisting of a semiconductor material, generally germanium or silicon, and used for rectification, amplification and switching. Its mode of operation utilizes transmission across the junction of the donor electrons and holes.